In communications networks, there may be a challenge to obtain good performance and capacity for a given communications protocol, its various design aspects and the physical environment in which the communications network is deployed.
For example, one design aspect with a considerable impact on performance and capacity for a given communications protocol in a communications network is the use of reference signals (RSs). RSs of different types can he transmitted, received, and used within an orthogonal frequency-division multiplexing (OFDM) symbol.
In addition to RSs, there are basically two types of physical downlink control channels (PDCCHs) envisioned for future radio access technologies; common PDCCHs and device-specific PDCCHs. The PDCCHs may be transmitted in a common control region or a device-specific control region.
In the 3GPP Long Term Evolution (LTE) suite of telecommunication standards, a search space may be understood as a set of candidate control channels which a wireless device is supposed to attempt to decode. There may be more than one search space. In particular, a search space may be a common search space, which is common to all wireless device of the cell, or a device-specific search space, which may have properties determined by a non-injective function of device identity and may thus be shared with some other devices of the cell. In a LTE cell, all search spaces may be contained in a constant set of one or more subbands.
For the PDCCH in 3GPP Rel. 8, the common control region (structured as a common control search space) is located within the protocol layer-1/layer-2 (L1/L2) control regions in the first few OFDM symbols spanning the entire system bandwidth, as well as any device-specific control regions (structured as device-specific search space(s)). In addition, common reference signals (CRS) are transmitted in the entire subframe (including the L1/L2 control region). Any PDCCH in the common or device-specific search space(s) are transmitted using the same antenna weights (heamforming) as the CRS.
The wireless device monitors the common and the device-specific search spaces in respective control regions and uses the CRS to estimate a channel, in order to do blind decoding of possible PDCCH candidates in the search spaces. This prevents device-specific beamforming of any device-specific PDCCHs, since the CRSs are not assumed to be beamformed in a device-specific way. Many of the PDCCH messages are not addressed to individual wireless devices but to a group of wireless devices, for example, random access responses, system information, allocation and paging information.
In 3GPP Rel. 11, a new set of device-specific control channel search space(s) were added along with related device-specific demodulation reference signals (DMRS). This enables the network to send device-specific control messages to a wireless device using device-specific beamforming, for example directed towards a certain wireless device or a certain group of wireless devices. Search spaces known as ePDCCH search spaces (where the prefix e- is short for enhanced) are located in a control region sent (and received) after the L1/L2 symbols in the data region, and are confined to a small subset of resource blocks.
FIG. 1 schematically illustrates an example of a structure of a 3GPP Rel. 11 subframe 150 showing frequency usage (in terms of bandwidth) as a function of time. The subframe 150 comprises a PDCCH control region 190, a data region 170 and an ePDCCH control region 180, where the ePDCCH control region 180 comprises an ePDCCH 160. The ePDCCH 160 may carry control information scheduling a data region 170 in the same subframe. The wireless device monitors the ePDCCH in the one or more ePDCCH search spaces 180. If an ePDCCH 160 is found, the found ePDCCH may identify a data region 170 in the subframe. It follows from FIG. 1 that the decoding of any data in the scheduled data region cannot be started until the ePDCCH region has been fully monitored, that is, after the entire subframe has been received. There may as well be deinterleaving.
Hence, there is a need for an ved monitoring in search spaces.